This invention pertains to tilted gratings in single mode waveguides, typically a single mode optical fiber, and to optical communication systems that comprise such gratings.
Bragg gratings in single mode waveguides typically couple a forward core-guided mode to backreflected modes in the core and the cladding. In at least some cases, it is desirable to control the relative strengths of these couplings to achieve a desired function. For instance, the coupling to backward-propagating cladding modes in single mode fibers may be used in loss filters. This typically requires that the undesirable core mode reflection be minimized in comparison to the cladding mode coupling.
The mode coupling strengths of gratings generally depend on the waveguide photosensitivity profile and the electric field of a given mode, both of which are largely fixed at the time of grating formation. A grating parameter which can be changed to alter the relative strength of the mode couplings is the tilt of the grating with respect to the waveguide axis. However, in prior art waveguides the degree of control that is achievable by means of the grating tilt is quite limited. For instance, in prior art single mode waveguides, the angular range of the tilt angle xcex8 over which a given mode coupling substantially is zero (defined herein as less than xe2x88x9230 dB) is quite limited, typically 0.1xc2x0 or less. Such gratings are difficult to manufacture.
Tilted gratings in optical fiber are known. See, for instance, U.S. Pat. No. 5,740,292, which discloses tilted refractive index gratings for coupling light in a fundamental mode (e.g., LP01) into a higher order mode (e.g., LP11). Such a grating can find a variety of uses, e.g., as a wavelength-dependent loss element with abrupt wavelength dependence. See also U.S. Pat. No. 5,832,156, which discloses a tilted grating in a dispersive optical waveguide tap.
Thus, there exists a need for a tilted waveguide grating in a single mode waveguide that can provide a broader tilt angle range of substantially zero coupling into the backwards core mode. This application discloses such a tilted waveguide grating. Furthermore, prior art tilted Bragg gratings typically have relatively low cladding loss (typically substantially less than 20 dB) as well as relatively low bandwidth (typically substantially less than 20 nm). However, there is a need for tilted Bragg gratings in single mode optical waveguides that not only are readily manufacturable but that also have relatively large cladding loss (e.g.,  greater than 20 dB) and relatively large bandwidth (e.g.,  greater than 20 nm). Such gratings can, for instance, advantageously be used in Er-doped fiber amplifiers to reject undesired ASE (amplified spontaneous emission). See, for instance, R.P. Espindola et al., paper WD4, xe2x80x9cOptical Amplifiers and Their Applicationxe2x80x9d (OAA), 1999 Nara, Japan.
Prior art tilted Bragg grating filters in fibers with complex radial photosensitivity profile typically have a photosensitive cladding. For instance, M. J. Holmes et al., ECOC ""98, September 1998, Madrid, Spain, pages 137-138, disclose sidetap filters that comprise a tilted Bragg grating in single mode fiber. The fiber had a non-photosensitive core dopant, and a photosensitive cladding doped with germania, to which boron was added in order to reduce the cladding refractive index to match the deposition tube. The fiber thus had a conventional refractive index profile, with the core refractive index greater than the cladding index, but had zero photosensitivity in the core and a non-zero photosensitivity in the cladding. See FIGS. 1a and 1b below. See also E. Delevaque et al., Optical Fiber Communication Conference 1995, Paper PD5; C. W. Haggans et al., IEEE Photonics Technology Letters, Vol. 10(5), May 1998, page 690; I. Riant et al., Optical Fiber Communication Conference 1999, Paper ThJ6-1/147; L. Dong et al., Bragg Gratings, Photosensitivity and Poling in Glass Waveguides Conference, 1999, Paper PD3; and L. Brilland et al., Electronics Letters, Vol. 35(3), Feburary 1999, page 234.
The above-cited Delevaque and Dong papers describe fiber designs in which core and cladding photosensitivity are adjusted to reduce cladding mode loss in untilted gratings.
M. J. Holmes et al., ECOC ""99, Sep. 26-30, 1999, Nice, France, pages I-216-217 disclose a fiber for sidetap filters. The fiber had a non-photosensitive core dopant for normalized radius less than 0.4, a combination of a non-photosensitive core dopant and germania for normalized radius 0.4-1, and a photosensitive cladding doped with germania out to a normalized radius of 3.5, to which boron was added to reduce the cladding index to match the deposition tube. The germania concentrations for the regions 0.4-1.0 and 1.0-3.5 were in the ratio 0.6-1 in order to obtain the required relative photosensitivity. See FIGS. 2a and 2b below. The above cited 1999 Holmes et al. paper thus discloses fiber in which the core has two different photosensitivity levels, with the cladding also being photosensitive. The photosensitivity profile was chosen to optimize the wavelength dependence of the cladding mode loss spectrum for gain flattening filter applications. The Bragg grating in this fiber is limited to applications as a weak narrow bandwidth filter. Thus, there exists a need for a fiber grating that can readily be made to have very low core mode reflection, and that has large cladding mode loss (preferably greater than 20 dB) over a large bandwidth (preferably greater than 20 nm). Practice of the present invention is advantageous in prior art narrow bandwidth applications because it will provide an even lower level of core mode reflection.
For ease of exposition the discussion herein will generally refer to optical fibers. It will be appreciated, however, that similar results are obtainable in other optical waveguides, e.g., in planar waveguides.
By a xe2x80x9cregular nullxe2x80x9d we mean herein a tilt angle region in a tilted (xe2x80x9cblazedxe2x80x9d) fiber Bragg grating that has a core mode coupling for light of a predetermined wavelength that is less than xe2x88x9230 dB over only a small (typically less than 0.1xc2x0) angular range of the tilt angle. See, for instance, FIGs. 1a and 1b. Regular nulls occur for many tilt angles.
By a xe2x80x9csuper nullxe2x80x9d we mean two (or possibly more) regular nulls that occur at closely spaced blaze angles, thereby making the core mode coupling at the predetermined wavelength very low (typically less than xe2x88x9230 dB) over a relatively large (more than 0.1xc2x0, desirably more than 0.2xc2x0, or even 0.50 or more) range of tilt angles between the regular nulls.
Modes of the guided light are designated LPmn. in conventional fashion, with m and n being integers. For instance, LP01 is the fundamental mode. LP01,f refers to the forward propagating fundamental mode, and LP01,b refers to the backward propagating fundamental mode.
xe2x80x9cPhotosensitivityxe2x80x9d refers to the refractive index change in the waveguide that results if an appropriately doped waveguide is exposed to actinic radiation, typically UV radiation.
By xe2x80x9ccladding mode lossxe2x80x9d we mean herein waveguide loss which results from grating coupling of a core guided mode to cladding modes.
By the xe2x80x9cband widthxe2x80x9d of a tilted Bragg grating we mean herein the wavelength interval over which the cladding loss is greater than 3 dB.
In a broad aspect the instant invention is embodied in an article that comprises a tilted Bragg grating of novel design in a single mode waveguide, the tilted grating selected to provide a relatively large (exemplarily  greater than 0.1xc2x0) range of tilt angle xcex8 wherein there is  less than xe2x88x9230 dB coupling of radiation of predetermined wavelenght xcex from a forward propagating core mode (e.g., LP01,f) into a backward propagating core mode (e.g., LP01,b), whereby manufacture of the tilted grating is facilitated. The relatively large range of tilt angle constitutes a xe2x80x9csuper nullxe2x80x9d, achieved through appropriate choice of the photosensitivity profile of the fiber core.
Furthermore, preferred Bragg gratings according to the invention have a relatively large cladding loss, typically in excess of 20 dB, and have relatively large bandwidth, typically greater than 20 nm, also attained through appropriate choice of the photosensitivity in the core. Frequently, but not necessarily, there is no photosensitivity in the cladding.
More specifically, the invention is embodied in an article that comprises a tilted refractive index grating in an optical waveguide, typically a single mode optical fiber. The grating has a tilt angle xcex8 and extends longitudinally over at least a portion of the single mode optical waveguide. The waveguide has a core and a cladding that surrounds the core, the waveguide has a dopant distribution selected to provide a radial refractive index. profile n(r) and a radial photosensitivity profile p(r), where n and p are the refractive index and the photosensitivity, respectively, and r is the radial coordinate of the optical waveguide, where p(r) varies as a function of r in the core, with p(r) having at least two different levels of photosensitivity in the core, said tilted refractive index grating having a coupling constant xcexa(xcex8) that determines the coupling between a forward propagating core mode (e.g., LPoI,f) of wavelenght xcex and a reflected core mode (e.g., LP01,b) of wavelength xcex.
Significantly, the photosensitivity profile p(r) is selected such that p(r) has at least one tuning region of zero or low photosensitivity, and n(r) and p(r) are selected such that xcexa(xcex8) is less than xe2x88x9230 dB over a range of xcex8 that is more than about 0.1xc2x0, preferably more than 0.2xc2x0 or even 0.5xc2x0, whereby manufacture of a tilted refractive index grating having substantially no reflected core mode at wavelenght xcex is facilitated. In preferred embodiments p(r) is selected such that the grating has a cladding loss in excess of 20 dB and a bandwidth in excess of 20 nm.
A feature of the invention is doping such that the refractive index profile n(r) differs from the photosensitivity profile p(r). As those skilled in the art know, doping of silica with Ge or P increases the refractive index and provides photosensitivity for ultraviolet (UV) radiation. On the other hand, doping of silica with Al increases the refractive index but does not provide photosensitivity, and doping with F or B reduces the refractive index and does not provide photosensitivity, although B may enhance the photosensitivity of other dopants. Other dopants may also be useful for tailoring n(r) and p(r).
It will be understood that appropriate choice of n(r) and p(r) results in a supernull in the coupling constant for the predetermined wavelength xcex. The presence of the supernull makes the tilted grating relatively easy to manufacture, as compared to prior art gratings wherein p(r) is proportional to n(r), and which consequently have only regular nulls.